This invention relates to light emitting matrix array display devices with light sensing elements. More particularly, the invention is concerned with a matrix array display device comprising an array of addressable pixels comprising light-emitting display elements, and light sensing elements. The invention is concerned especially, but not exclusively, with matrix display devices using electroluminescent display elements, particularly organic electroluminescent display elements, OLEDs, including polymer electroluminescent elements, PLEDs.
An example of matrix display device whose pixels comprise electroluminescent (EL) display elements and light sensing elements is described in British Application No 0005811.5. The described device comprises an active matrix display device having an array of pixels carried on a substrate, in which each pixel includes a current-driven electroluminescent display element comprising light emitting EL material between two electrodes, one of which is transparent, and a switching device operable to control the current through the display element, and hence its light output, in a drive period based on a drive (data) signal applied to the pixel in a preceding address period.
As in other active matrix EL display devices, such as the device described in EP-A-0717446, the display elements, which need to continuously pass current in order to generate a light output, can be energised for an extended period, up to a frame time, following the addressing of the pixel in a respective row address period with the level of the data signal stored in the pixel in the address period determining its output during this drive period. The driving device, in the form of a thin film transistor (TFT), is responsible for controlling the current through the display element and the applied data signal is stored as a charge on a capacitance coupled to the gate of this drive TFT so that the operation of the TFT is dependent on the stored charge.
The pixels in the device of British Patent Application No 0005811.5 further include a thin film photosensitive device, comprising a (PiN) photodiode or a photo-responsive TFT coupled to the storage capacitance that is arranged in operation of the pixel to be reverse biased and is responsive to light emitted by the pixel""s display element in the drive period so as to leak charge from the capacitance at a rate dependent on the display element""s light output level. Thus, by virtue of the photo sensitive device, opto-electronic feedback is provided which progressively adjusts the operation of the drive TFT controlling energisation of the display element during the drive period to reduce the current flow through the display element, and hence its light output, by progressively discharging the capacitance (assuming it is charged upon addressing). The proportion of the total available drive period for which the display element is energised is, therefore, dependent on, and regulated by, this feedback arrangement according to the element""s light output. In this way the integrated light output from a display element in a drive (frame) period can be controlled so as, inter alia, to counteract any effects of ageing or degradation in the display element""s electroluminescent material, particularly a reduction in light output level for a given drive current level, which can occur over a period of time of operation, and also to compensate for the effects of voltage drops occurring in current carrying lines supplying the pixels.
Such a technique is valuable in achieving a high quality display by ensuring that pixel light outputs can be constant and uniform over time. However, the implementation of such a pixel circuit can be problematic. The photocurrent generated by the photosensitive device needs to be very small in order to appropriately control the TFT gate potential over a frame period if the use of a large storage capacitance is to be avoided. Also, the provision in each pixel circuit of the light sensitive element using thin film technology ideally should not unduly complicate fabrication while at the same time good optical coupling between the light-emitting display element and the light sensing element needs to be ensured.
According to the present invention, there is provided a. light emitting display device comprising on a substrate an array of addressable pixels each comprising a light-emitting display element having a layer of light-emitting material with a transparent electrode on one side thereof, and a light sensing element responsive to light emitted by the display element, wherein each light sensing element comprises a gated photosensitive thin film device comprising a semiconductor layer having contact regions laterally spaced on the substrate and an intervening gate controlled region over which dielectric material is disposed, and wherein a part of the light-emitting display element extends over the dielectric material and the gate controlled region such that the transparent electrode of the light-emitting display element at that part serves as the gate of the photosensitive device and light emitted by the light-emitting material layer is incident on the semiconductor layer.
With this arrangement, the provision of the light sensing element is relatively uncomplicated while at the same time good optical coupling between this component and the light-emitting material of the display element is reliably ensured. Such an arrangement, therefore, is highly beneficial when used, for example, in the kind of display device described in the aforementioned application No 0005811.5. The basic structure of the gated photosensitive device is generally similar to that of TFTs commonly used in matrix display devices, e.g. the driving TFTs of the aforementioned display device. In this respect, the gated photosensitive device may comprise a TFT or a gated lateral (pin) diode. The provision of this component is, therefore, entirely compatible with the thin film technology used for matrix display devices and the device can easily be formed simultaneously with such TFTs from common thin film layers. Because a part of the light-emitting element extends over the photo device and the active region of the device is directly and closely associated with the light-emitting material, then the device is highly responsive to variations in light output from the light-emitting material. Moreover, as a part of the electrode of the light-emitting element is utilised for the gate of the photosensitive device, then the gate potential of this device, corresponding to the potential of this electrode in operation of the device, can conveniently be biased appropriately during operation such that it behaves as a photosensitive leakage device by virtue of photocurrents generated therein in response to light incident from the light-emitting layer in the manner required for this component when used as a charge adjusting device in the display device of the aforementioned type.
While the invention is particularly beneficial in the implementation of the particular kind of pixel circuit discussed above, it is envisaged that it can be used to advantage in other light emitting matrix display devices in which pixels include a light sensing device responsive to the light emission of the pixel""s display element for another purpose rather than being used in the particular manner described.
The light-emitting elements preferably comprise electroluminescent elements, such as OLED or PLED elements. However, it is envisaged that the invention could be used to similar advantage in display devices using other kinds of light-emitting elements and not necessarily in the case of the light sensing element being used as part of an electro-optic feedback arrangement in the manner described.